Heat exchangers used in furnaces and other heating apparatus are typically comprised of plural metal tubes, each of which may be bent in a serpentine fashion to form multiple passes for the flue gas flowing in each tube. The inlet of each tube is in communication with a burner assembly in which a combustible fuel-air mixture is burned. The outlet of each tube is in communication, either directly or indirectly through a secondary heat exchanger, with a flue vent or the like, whereby flue gas is exhausted from the heating apparatus. The flue gas flowing in the heat exchanger tubes transfers heat to air passing over the outside of the tubes, whereby air supplied to an indoor space is heated.
It is known in the art that heat transfer efficiency may be enhanced by slowing the flow of the gaseous products of combustion in the tubes and by increasing the turbulence thereof. One approach to accomplishing both of these results is to insert one or more baffles in the tubes to break up the laminar flow of the hot gas. Another approach is to flatten the tubes at certain locations to restrict and alter the flow of the gas. Yet another approach, as described in published U.S. patent application US 2002/0005275, is to extrude opposing pairs of dimples into the tube, so that the dimples of each pair are in alignment and form a pair of adjacent converging, diverging nozzles inside the tube.